The invention relates to a method for operating a brushless direct current motor and to a brushless direct current motor.
In a method of this type, by the energization of a plurality of armature coils which are arranged on a stator, a rotating field is generated in order to drive a rotor, which is rotatable about an axis of rotation relative to the stator and has at least two opposing permanent magnet poles. The armature coils form a three-phase current winding for generating a rotating field which rotates around the stator, and have three or more terminals, by means of which the armature coils are energized.
In brushless direct current motors, a distinction is drawn between sensor-controlled commutation and sensorless commutation. Generally, in brushless direct current motors, the circumferential armature field, which is generated at the stator, is electronically commutated according to the rotor position, the rotor speed and rotor torque. Electronic commutation can thus be employed to regulate the operational performance of the direct current motor.
In sensor-controlled commutation (for “sensor-controlled brushless direct current motors”), sensors such as, for example, Hall effect sensors for the detection of the magnetic flux of the rotor, or optical sensors, are located in the region of the stator. The sensors deliver information on the rotor position, which is thus sensor-detected. Electronic commutation can be adjusted, according to the sensor-detected rotor position.
In sensorless commutation (for “sensorless brushless direct current motors”), conversely, detection of the rotor position is achieved by means of the negative field voltage, which is induced in the armature coils of the stator, can be evaluated by the control device for the determination of the rotor position, and is also described as the counter-e.m.f. (e.m.f.: induced electromagnetic force).
However, the determination of this counter-e.m.f. is only possible with effect from a minimum rotor speed, on the grounds that, below this minimum speed, the induced counter-e.m.f. is too small. Consequently, conventional brushless direct current motors have provided for the orientation of the rotor according to a specified switching model, followed by the off-load switching-in of phases for the purposes of start-up until a minimum speed is achieved, and the angular position of the rotor can be determined from the induced counter-e.m.f.
A method is therefore desirable whereby, in a sensorless brushless direct current motor, even at rest or at a low rotor speed, the angular position of the rotor can be determined, in order to start-up the motor in a reliable and efficient manner.
In a sensorless brushless direct current motor known from WO 2009/053388 A2, it is provided that, for the determination of the rotor position at rest, a measuring sequence is applied to two of three motor terminals, and a voltage is measured on the respective third, non-energized motor terminal. This voltage is compared with a reference voltage and, on the basis of the comparison, the angular range within which the rotor currently lies can be determined.
The method according to WO 2009/053388 A2 makes it possible to determine in which of six potential angular ranges a rotor is currently located. This is achieved by the application of measuring pulses in the context of a measuring sequence. However, the accurate angular determination of the rotor position at rest, during the rotation of the rotor or during block commutation by the method according to WO 2009/053388 A2 is not possible, or only possible to a limited extent.